A low-level event which indicates that a container's contents
changed because a component was added or removed.

Container events are provided for notification purposes ONLY;
The AWT will automatically handle changes to the containers
contents internally so that the program works properly regardless of
whether the program is receiving these events or not.

This low-level event is generated by a container object (such as a
Panel) when a component is added to it or removed from it.
The event is passed to every ContainerListener
or ContainerAdapter object which registered to receive such
events using the component's addContainerListener method.
(ContainerAdapter objects implement the
ContainerListener interface.) Each such listener object
gets this ContainerEvent when the event occurs.

An unspecified behavior will be caused if the id parameter
of any particular ContainerEvent instance is not
in the range from CONTAINER_FIRST to CONTAINER_LAST.